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A 4-month-old male Jack Russell terrier was evaluated for non-painful muscle spasms and collapse associated with exercise and activity. Clinical examination revealed well-defined, non-painful hypertrophic muscles of the fore and hind limbs and exercise and excitement induced hindquarter bunny-hopping gait, which improved with activity but worsened with resting and with any sudden changes in direction of movement. Neurological examination and routine laboratory testing showed no abnormalities. DNA analysis for myotonia congenita showed the dog to have a gene mutation in the chloride ion channel, diagnostic for myotonia congenita, which has not been reported in the Jack Russell terrier breed.